In storage systems an array of independent storage devices can be configured to operate as a single virtual storage device using a technology known as RAID (Redundant Array of Independent Disks). A computer system configured to operate with a RAID storage system is able to perform input and output (I/O) operations (such as read and write operations) on the RAID storage system as if the RAID storage system were a single storage device. A RAID storage system includes an array of independent storage devices and a RAID controller. The RAID controller provides a virtualised view of the array of independent storage devices—this means that the array of independent storage devices appear as a single virtual storage device with a sequential list of storage elements. The storage elements are commonly known as blocks of storage, and the data stored within them are known as data blocks. I/O operations are qualified with reference to one or more blocks of storage in the virtual storage device. When an I/O operation is performed on the virtual storage device the RAID controller maps the I/O operation onto the array of independent storage devices. In order to virtualise the array of storage devices and map I/O operations the RAID controller may employ standard RAID techniques as discussed in the paper “A Case for Redundant Arrays of Inexpensive Disks (RAID)” (D. Patterson et. al., 1988). Some of these techniques are considered below.
In providing a virtualised view of an array of storage devices as a single virtual storage device it is a function of a RAID controller to spread data blocks in the virtual storage device across the array. One way to achieve this is using a technique known as Striping. Striping involves spreading data blocks across storage devices in a round-robin fashion. When storing data blocks in a RAID storage system, a number of data blocks known as a strip is stored in each storage device. The size of a strip may be determined by a particular RAID implementation or may be configurable. A row of strips comprising a first strip stored on a first storage device and subsequent strips stored of subsequent storage devices is known as a stripe. The size of a stripe is the total size of all strips comprising the stripe. The use of multiple independent storage devices to store data blocks in this way provides for high performance I/O operations when compared to a single storage device because multiple storage devices can act in parallel during I/O operations.
Physical storage devices such as disk storage devices are renowned for poor reliability and it is a further function of a RAID controller to provide a reliable storage system. One technique to provide reliability involves the storage of check information along with data in an array of independent storage devices. Check information is redundant information that allows regeneration of data which has become unreadable due to a single point of failure, such as the failure of a single storage device in an array of such devices. Unreadable data is regenerated from a combination of readable data and redundant check information. Check information is recorded as parity data which occupies a single strip in a stripe, and is calculated by applying the EXCLUSIVE OR (XOR) logical operator to all data strips in the stripe. For example, a stripe comprising data strips A, B and C would be further complimented by a parity strip calculated as A XOR B XOR C. In the event of a single point of failure in the storage system, the parity strip is used to regenerate an inaccessible data strip. For example, if a stripe comprising data strips A, B, C and PARITY is stored across four independent storage devices W, X, Y and Z respectively, and storage device X fails, strip B stored on device X would be inaccessible. Strip B can be computed from the remaining data strips and the PARITY strip through an XOR computation. This restorative computation is A XOR C XOR PARITY=B.
For check information to be effective in the event of a failure it is necessary that it is accurate and maintained. Changes to data in a RAID storage system must therefore be reflected by appropriate changes to check information. This can be burdensome where changes to data affect a data unit smaller than the size of an entire stripe (known as a “small write”) for the reason described below. Consider a RAID storage system with a RAID controller using the striping technique, with each stripe comprising data strips and a parity strip. If such a system is configured such that a single block in a virtual view of the storage system (a virtual storage device) corresponds to a single data strip, a write operation of a single block to the virtual storage device is implemented by the RAID controller as a write operation of a single strip in a RAID stripe. The change of a single strip in a stripe must be reflected by appropriate changes to a corresponding parity strip. Thus the parity strip for the stripe must be recalculated. To recalculate the parity strip, the data strip being overwritten in the stripe (the “old” data strip) must be excluded from the existing parity data. This can be achieved by performing an XOR operation on the parity data and the old data strip. Furthermore, the replacement data strip (the “new” data strip) must be included in the parity data to create a modified parity strip. This can be achieved by performing an XOR operation on the parity data and the new data strip. Subsequently, the modified parity strip and the new data strip must be written within the stripe. Thus in order to write a replacement strip to an existing stripe it is necessary to perform the following operations: two read operations (of the old data strip and the parity strip); a modification of the parity strip; and two write operations (of the new data strip and the modified parity strip). This approach to conducting a small write leads to a reduction in performance of a RAID storage system due to additional read and write operations required to maintain the consistency of check information. This is known as the read-modify-write problem for small write operations.
Due to the performance implications of small-write operations outlined above it is preferable to overwrite one or more complete stripes when writing to a RAID storage system to avoid the need to maintain existing check information. In order for a write operation to overwrite one or more complete stripes it is necessary that a unit of data to be written to the RAID storage system is of an appropriate size, being a multiple of the stripe size (excluding parity strips). To overwrite one or more complete stripes it is also necessary that a unit of data is written to a block in the virtual view of a RAID storage device corresponding to a first strip in a RAID stripe. A write operation to a block corresponding to any strip other than a first strip in a RAID stripe would include the writing of part of a stripe which is a small-write operation. Ensuring that a write operation corresponds to a first strip in a RAID stripe is known as “stripe alignment”. Thus to ensure a write operation completely overwrites one or more RAID stripes it must meet stripe size and alignment criteria for a given RAID storage system. A write operation which satisfies these criteria is known as a “stripe aligned write”. Stripe aligned writes do not require the maintenance of existing check information in RAID stripes because one or more stripes are completely replaced with newly written data and new check information is calculated for this new data as part of the write operation. Ensuring all write operations to a RAID storage system are stripe aligned improves performance by removing the read-modify-write problem of small-writes.
Write operations to a RAID storage system will not always be stripe aligned, and small write operations will never be stripe aligned as by definition a small write operation involves a data unit smaller in size than a complete RAID stripe. Existing RAID storage systems may employ a memory, such as a cache, in which multiple small write operations are collected into a single write operation constituting a complete stripe. Subsequently a collection of small writes can be stripe aligned and written to a RAID storage device. For such a cache memory to be effective it must be configured to operate within the parameters of a RAID storage system including the stripe size and alignment. These parameters may be different for different RAID storage systems, and are typically different for RAID storage systems provided by different vendors. Often cache memory is integrated within a RAID controller so that it is easily configured with appropriate stripe size and alignment parameters by a user or by the controller itself.
Increasingly, RAID storage systems are themselves becoming virtualised in configurations such as storage area networks (SANs). A SAN comprises a network linking one or more servers to one or more storage devices. Storage devices in a SAN may include virtual storage devices implemented as RAID storage systems. Within a SAN one or more switches connect devices and provide routes through the SAN between hosts and storage devices. A SAN virtualises storage devices to ensure interoperability of devices connected to the SAN. It is a feature of the virtualisation of storage devices in a SAN that actual implementation details of a storage device may be unknown to other devices in the SAN. For example, hosts in a SAN may be unable to determine whether a storage device attached to the SAN is implemented as a single disk or a RAID storage system. The virtualisation of storage devices in a SAN also allows the spreading of data across many storage devices, including many RAID storage systems, to further improve performance and reliability. This is achieved using a storage appliance, such as IBM's TotalStorage Virtualization Engine, which can be attached to a switch in a SAN. It is desirable that cache memories are not integrated within storage devices such as RAID storage systems but are implemented independently within the SAN so that they can be used when data is spread across multiple SAN storage devices. For example, cache memory may be implemented within a storage appliance attached to a switch within the SAN.
Where a cache memory is not integrated with a RAID controller of a RAID storage system, and the RAID controller is virtualised such as through a SAN, specific stripe size and alignment characteristics of the RAID storage system are not readily available to the cache memory. The separation of a cache memory from a virtualised RAID controller leads to the problem that the cache memory cannot be automatically configured with appropriate stripe size and alignment characteristics because these characteristics are not known to the cache memory, and configuration must take place manually or not at all. The consequences of an inappropriate stripe size and alignment configuration include the increased likelihood that the read-modify-write problem is encountered as write operations are unlikely to be properly stripe aligned. It would thus be desirable to provide a system and method to alleviate these problems with conventional storage systems.